Its really simple, apply the affects of the game they left to their rating even after they quit. If they quit the game and their team loses the game then they still have a loss on their record. That way leaving has no affect on them other than increasing the chances of a loss on their record. Now players are encouraged to stay because if they dont then their team will only have a greater chance of losing. With this method every game you join you are responsible for, theres no getting out even if you quit.
Sure some people would leave to make sure their K/D doesnt continue to fall but at this point the amount of quitters will be small enough to increase the number of games a quitter can leave before a ban without affecting the rest of the community and then people with disconnects and server issues can rest in peace.
Whatre your thoughts or problems with this idea?